Your Beating Heart
Make a fist with your hand. This is about the size of your heart. Now tighten your fist and let go. Do this about twenty times. This action is similar to the way a heart beats. Is your hand tired? The good thing about a healthy heart is that it doesn’t get tired. Day in and day out, your heart keeps beating.
About half your blood is water that contains dissolved sugar, salt, and other chemicals. The rest is made up mostly of disease-fighting white blood cells, sticky pieces of cells called platelets, and many millions of red blood cells.
Iron in red blood cells gives human blood its red color. But not every animal has red blood. Examine a lobster’s blood, and you’ll see that it’s blue. That’s because it doesn’t have iron in it.
Lobster
How Blood Circulates
Your heart pumps blood throughout your body. With every heartbeat, blood surges out of your heart and into tubes called blood vessels. When blood leaves the heart, it enters blood vessels called arteries. The arteries divide into smaller and smaller blood vessels that carry blood to all parts of the body. The smallest kind of blood vessel in your body is the capillary. Capillaries are so small that you need a microscope to examine them. Blood vessels called veins carry blood back to your heart.
Blood always circulates around your body in the same direction. Blood leaves the heart, travels to each of the cells in your body, and returns to the heart with much less oxygen. The heart then pumps this blood to the lungs to get more oxygen. The oxygen-rich blood travels back to the heart, and the cycle begins all over again. Your heart and blood vessels make up your circulatory system.